Photonic band gap crystals are artificially created structures in which the index of refraction is periodically modulated in 1D, 2D, or 3D, with period of modulation comparable to the wavelength of the carrier light. For a 2D example, see FIG. 1A. FIG. 1A is a schematic diagram of a 2D PBG crystal 100, in which the dark circles 102 represent regions of a high index of refraction.
If the amplitude of the index variation is large enough, a photonic band gap appears, and light of certain wavelength ranges is not allowed to propagate. In many ways, PBG crystals present for photons what semiconductors present for electrons. Therefore, they are considered to be a very promising direction for building optical integrated circuitry. In an integrated optical circuit, the carrier light would conceivably be guided down line defects in PBG crystals. Exemplary line defects are illustrated in FIGS. 1B and 1C.
FIG. 1B is a schematic diagram of a line defect 104 in the 2D PBG crystal 100. FIG. 1C is a schematic diagram of another type of line defect 106 in the 2D PBG crystal. The signal propagates down the line defect 106 by hopping between the weakly coupled cavities. In such defects, light can be easily made to bend around a comer (like in FIG. 1D), or split equally into two branches with no reflection backwards. FIG. 1D is a schematic diagram of a line defect 108 in the 2D PBG crystal 100 that bends light at 90°. 100% transmission can be obtained for certain carrier frequencies around such bends. Furthermore, add-drop filters for light of different carrier frequencies have also been proposed.
All the effects described heretofore are passive. Once a device has been built, its properties cannot be altered. Nevertheless, it would be useful to have a device whose properties can be altered with some external stimulus: electrical, optical, or mechanical. Some proposals in this direction have already been made. The invention provides a class of fast (up to more than O(1 THz)), compact, wide bandwidth, small power devices, which can be operated with electrical or optical stimuli, and are easy to implement in many PBG crystals. In order to achieve these desirable specifications, properties of some especially designed line defects in PBG crystals are used.